1. Field of the Invention
This invention relates to a diaphragm device for use in a photo-taking lens or the like.
2. Related Background Art
In one form of a diaphragm device used in a photo-taking lens or the like, a plurality of diaphragm blades are disposed about the optic axis of a diaphragm opening to cover the surroundings of the diaphragm opening and thereby limit a light passing through the diaphragm opening, and further the diaphragm blades may be pivotally rotated at a time, whereby the quantity of light passing through the diaphragm opening can be changed.
Now, it is desirable that the shape of the diaphragm opening (the surroundings of the diaphragm opening defined by the inner edge portions of the diaphragm blades) be generally approximate to a circular shape. For example, during photographing in the daytime, the shape of the diaphragm opening greatly affects the degree of vignetting of the background and as the diaphragm opening becomes more non-circular, the degree of vignetting becomes greater, that is, vignetting becomes worse. Also, during photographing in the nighttime, when a light source is photographed, the light source is vignetted into the shape of the diaphragm opening (a polygonal shape or the like) and imprinted on film. Accordingly, the vignetting of the background becomes worse, and to solve it, it becomes necessary to make the shape of the diaphragm opening approximate to a circle to the utmost irrespective of the amount of aperture.
FIG. 4 of the accompanying drawings is an enlarged view of one of diaphragm blades 10 used in the diaphragm device according to the prior art. FIGS. 5A to 5C of the accompanying drawings show a state in which the diaphragm blades 10 as shown in FIG. 4 are disposed around a diaphragm opening and the diaphragm blades overlap one another when the diaphragm is progressively stopped down from the fully open side to the small aperture side, and FIG. 5D of the accompanying drawings is an enlarged view of the diaphragm opening.
In the diaphragm blade 10 used in the prior-art diaphragm device, as shown in FIG. 4, the portion forming the diaphragm opening, i.e. the inner edge portion 10a of the diaphragm blade 10, is usually formed by a single arc having a fully open aperture or greater. However, in such a diaphragm blade, the marginal portion of the diaphragm opening certainly becomes approximate to a desirable circle in a state approximate to the fully open aperture, but when the diaphragm is stopped down as shown in FIGS. 5A to 5D, the diaphragm opening formed by the diaphragm blades becomes a polygon corresponding to the number of the diaphragm blades and often adversely affects the vignetting of the photograph taken.
On the other hand, in the prior-art diaphragm device constructed of the diaphragm blades 10, the angle of rotation of the blades becomes smaller toward the small aperture side. Therefore, the back-lash between a cam slot provided in a diaphragm blade operating member and pins provided integrally with diaphragm blades fitted therein, or the influence of their machining accuracy or the like upon the aperture diameter becomes relatively great, and this has led to the problem that even a slight back-lash negligible on the fully open aperture side makes the error of the aperture diameter great on the small aperture side.
Also, Japanese Utility Model Publication No. 45-29581 proposes diaphragm blades of the shape as shown in FIG. 6 of the accompanying drawings. FIG. 6 shows only two of the diaphragm blades described in Japanese Utility Model Publication No. 45-29581, and further shows the shape of the diaphragm opening when the diaphragm blades are stopper down, by broken lines. The diaphragm blade 100 has two sides 100a and 100b forming an angle .theta., and these two sides form the diaphragm opening. When the number of the sides of a polygon forming the diaphragm opening is n, the angle .theta. has a value EQU .theta.=(n-2).times.180.degree./n.
The diaphragm blade 100 is disposed so that the angle .alpha. it forms with the adjacent diaphragm blade 100' is .theta.=.alpha.. By adopting such a construction, the number of diaphragm blades necessary for forming the diaphragm opening by an n-polygon becomes n/2. Accordingly, even if the number of blades is the same as the number of blades in the prior-art diaphragm device, the polygon of the diaphragm opening formed thereby is twice in the number of sides and therefore, the shape of the diaphragm opening becomes more approximate to a circle.
Also, Japanese Utility Model Publication No. 36-20480 proposes a diaphragm device as shown in FIG. 7 of the accompanying drawings. FIG. 7 shows a pair of diaphragm blades and a cam mechanism in the diaphragm device described in Japanese Utility Model Publication No. 36-20480. The diaphragm blade 110 is of a shape similar to that of the diaphragm blade shown in FIG. 4, and the portion thereof forming a diaphragm opening is formed by a single arc. A cam slot 111 is provided in a diaphragm blade operating member, not shown. By the diaphragm blade operating member rotating about the optic axis O , the diaphragm blade 110 is stopped down. The cam slot 111 is comprised of a cam portion 111a in which a pin 112 provided integrally with the diaphragm blade is fitted to rotate the diaphragm blade, a slot portion 111b connected to the cam portion and provided near the cam portion, and a jetty portion 111c formed by the cam portion 111a and the slot portion 111b and having resiliency. By adopting such a construction, on the small aperture side, the pin 112 of the diaphragm blade is held down in the cam slot by the jetty portion 111c having resiliency and therefore, any back-lash between the cam slot 111 and the pin 112 can be eliminated. Accordingly, it becomes possible to increase the accuracy of the aperture diameter on the small aperture side.
Also, Japanese Laid-Open Utility Model Application No. 50-38735 proposes diaphragm blades of the shape as shown in FIGS. 8A and 8B of the accompanying drawings. FIGS. 8A and 8B show only one of the diaphragm blades of the diaphragm device described in Japanese Laid-Open Utility Model Application No. 50-38735, and FIG. 8A shows the fully open aperture state, and FIG. 8B shows the small aperture state, and in these figures, the letter 0 designates the optic axis. The portion forming a diaphragm opening is any curve which satisfies the following condition, i.e., such a curve that the angle .beta. formed by a straight line .gamma. drawn from the center of rotation 0' of the diaphragm blade to the point of contact O between the diaphragm blade and the diaphragm opening and the tangential line S of the diaphragm blade portion forming the diaphragm opening becomes greater toward the small aperture side. The amount of variation in the aperture diameter for the angle of rotation .theta. of the diaphragm blade is expressed by EQU O'P.multidot..theta..multidot.cos .beta..
Accordingly, on the small aperture side on which the angle .beta. is great, the angle of rotation .theta. of the diaphragm blade can be secured more greatly and the error due to the back-lash or the like between the cam slot and the pin can be made relatively small. Further, as regards the error of the position of that portion of the diaphragm blade which forms the diaphragm opening, the component in the direction of the normal to the aforementioned straight line r affects the aperture diameter, but as the angle 8 becomes greater, this amount becomes smaller, and on the small aperture side, the error itself can be made small.
Also, Japanese Laid-Open Patent Application No. 63-8638 proposes a circular diaphragm apparatus. The substance of this publication is that, of the inner edges of diaphragm blades for forming a diaphragm opening, the inner edge portion forming the diaphragm opening during medium stop-down on the pivotable free end side with respect to the inner edge portion forming the fully open diaphragm opening is formed into an arc along the circumference of a circle of a set radius smaller than a predetermined radius about the optic axis in the predetermined medium stop-down posture or an arc approximate thereto, whereby a medium aperture stopped down by one to two steps from the fully open aperture can be formed into a circle or a shape approximate thereto.
However, the constructions as described above have suffered from the following problems. First, the feature of the construction described in the aforementioned Japanese Utility Model Publication No. 45-29581 is that it intends to form a polygonal diaphragm opening having many sides by a small number of diaphragm blades and accordingly, the diaphragm opening is polygonal in both the fully open aperture and the small aperture, and it is difficult from the following point to achieve the purpose of making the shape of the diaphragm opening approximate to a circle by only this construction. That is, to make the shape of the diaphragm opening approximate to a circle, it is necessary to vary R of the corners of the two sides 100a and 100b in accordance with the radius of each aperture, but this is technically difficult. On the other hand, even if a construction is adopted in which the number of diaphragm blades is increased to thereby make the polygon of the diaphragm opening approximate to a circle, to ensure a regular polygon to be formed in each aperture diameter, it is necessary to move the center of rotation O' of each diaphragm blade relative to the optic axis, not shown, and this makes the mechanism very much complicated.
Next, in the cam slot of the structure as shown in FIG. 7 which is described in Japanese Utility Model Publication No. 36-20480, a jetty portion having resiliency is provided to eliminate back-lash, but the resiliency conversely results in weakened strength and the aggravation of the aperture accuracy by the deformation of the cam slot poses a problem. Further, there is also the disadvantage that on the small aperture side, the pin is held down and therefore an unnecessary frictional force is created to cause a reduction in the cam efficiency and this leads to the bad operability when a diaphragm operating ring, not shown, is extraneously operated.
Next, in the diaphragm blade as shown in FIGS. 8A and 8B which is described in Japanese Laid-Open Utility Model Application No. 50-38735, the portion which forms the diaphragm opening is constructed of a curve of a complicated shape and therefore, the polygon of the diaphragm opening becomes a more distorted shape as particularly the aperture diameter becomes greater, and the polygon is partly expanded or is partly broken, and this adversely affects the vignetting of photographs taken.
Next, in the diaphragm blades described in Japanese Laid-Open Patent Application No. 63-8638 a shape approximate to a circle is obtained for each one step before and after the medium aperture set on the fully open and the pivotable free end side, as described therein, but since many of lens barrels have six or more aperture steps from the fully open aperture to the small aperture, this method cannot realize a shape approximate to a circle in all the steps from the fully open aperture to the small aperture.
As noted above, any of the above-described examples of the prior art has been insufficient to solve the problems caused by the diaphragm blade as shown in FIG. 4 which has been used in the prior-art diaphragm device.